More than 100 trillion cells make up the human body. Most of these cells contain all the genes and other information needed to "build" a human being. Much of this genetic information is found in the nucleus of the cell, a "control center" that keeps all the material together in one place.
The nucleus stores its genetic material in packages called chromosomes. Most people have 46 chromosomes in each cell—23 from their mother and 23 from their father. After fertilization, the two sets of chromosomes match up to form 23 pairs. The chromosomes in the 23rd pair are called the sex chromosomes, X and Y; they determine a person's sex. Males usually have one Y chromosome and one X chromosome; females usually have two X chromosomes.
Each chromosome is made up of genes. Genes contain the information used by other parts of the cell to make proteins, the body's building blocks. Proteins make up the structure of your organs and tissues; they are also needed for your body's chemical functions. Each protein performs a specific job in different types of cells, and the information for making at least one protein is contained in a single gene.
Genes are made up of various lengths of DNA, which contains four chemicals: adenine (A), guanine (G), cytosine (C), and thymine (T). These chemicals line up like beads on a necklace to form strands of code. They also pair up with each other to form the double strands that are characteristic of DNA.
The gene's chemical code for a protein has two parts: the introduction area (promoter), and the instructions for creating the protein. The instructions for making the protein are inside the cell's nucleus, but the parts that actually make the protein are outside the nucleus. To send the instructions to the protein-producing areas of the cell, the gene "reads" the chemical code and rewrites it into a new form (called messenger RNA). The new form is then sent out of the cell's nucleus to make proteins. But, if either the original code or the new form of the code is incorrect or has missing parts, the cell can't make the correct protein. Without the protein, the body may not develop or function normally.
This is what happens in Fragile X. The FMR1 gene contains too many repeats of one specific sequence, CGG, which is an important part of the promoter region for making FMRP (see the figure below).
The number of CGG repeats affects how the code is read and rewritten into its new form. So when the new form of the code gets to the protein-making areas of the cell, the cell has trouble using it to make FMRP, the protein that is abnormal, missing, or in low amounts in people with Fragile X. The cell either can't use the code, and so makes no FMRP, or tries to use the code and makes abnormal or unusable FMRP.